How to seal boiler vent?

[Doron]

Hi

My boiler vents into a chimney and is not sealed (see pictures below). The guy from the gas company told me this could lead to carbon monoxide leak into the house.
Any suggestions on what's the best way to fix this?
Thanks

 

[TomFOhio]

Make up some mortar mix and cement around the hole by filling it in. Let it dry and then fill it in the rest of the
way until it is completely sealed.